The present invention relates to tape storage systems, and more specifically, to a high rate skew estimation for use in tape drives.
Timing-based servo (TBS) is a technology developed specifically for linear tape drives in the late 1990s. In TBS systems, recorded servo patterns include transitions with two different azimuthal slopes. An estimate of the head lateral position is derived from the relative timing of pulses generated by a servo reader reading the servo pattern.
In a TBS format, the servo patterns are prerecorded in several bands distributed across the tape, typically five or nine bands. Data is recorded in the regions located between pairs of servo bands. FIG. 3 illustrates the tape layout for five servo bands and four data bands, as specified in the linear tape-open (LTO) format and IBM Enterprise format. In read/write heads of IBM LTO and Enterprise tape drives, two servo readers are normally available per head module, from which longitudinal position (LPOS) information as well as a position error signal (PES) may be derived. Optimum detection of the TBS patterns is achieved by a synchronous servo channel employing a matched-filter interpolator/correlator, which ensures optimum filtering of the servo reader signal.
In drives using flanged rollers to guide the tape being transported from a supply reel to a take-up reel, the flanges limit the motion of the tape, but introduce a probability of debris accumulation on the tape. This debris accumulation over time impacts the lifetime of the tape and creates undesirable dynamic effects, namely high-frequency lateral tape motion (LTM).
One solution to this problem is to remove the roller flanges. By removing the roller flanges there is less constraint on the motion of the tape. Consequently, lateral tape motion is more pronounced, leading to large tape-to-head skew, but occurring at a significantly lower frequency than the LTM originated by debris accumulation, thus facilitating the task of a track-following system. To compensate for the skew resulting from LTM, skew-following actuation is used to keep the head perpendicular to the tape with a skew-following loop, and to enable read while write verification with a single track following actuator with two degrees of freedom and a multi-module write/read head.
One conventional method for the computation of a skew error signal (SES), which is defined as the difference between the actual skew and a reference skew, relies on the timing information that is provided by the peaks of the correlation signals, which are generated by two synchronous servo channels receiving readback signals from the two servo readers that read TBS patterns on adjacent servo bands. The method requires bringing the initial skew error signal for the skew-following loop to within a value of ±d equivalent to half a servo frame, corresponding, for example, to ±76 μm for one particular TBS servo format. This is possible due to the sequences of synchronization symbols that are encoded in the TBS patterns, interspersed with the LPOS symbols, and usually referred to as LPOS sync patterns. The LPOS sync patterns are the same in each servo band, as defined in both the LTO and IBM enterprise servo formats. For the purpose of initial alignment of the skew, so that the skew error is within ±d, the LPOS sync-pattern detection flags, which are generated by the two servo channels every 36 servo frames, are used to estimate the absolute skew. The block diagram of a joint track-following and skew-following servo system using dual synchronous servo channels is shown in FIG. 4. The operation of the skew-following loop, however, cannot rely solely on the presence of the LPOS sync patterns for the estimation of the skew, as the rate of generation of skew estimates would be too low.
In this conventional method the skew estimate is obtained from the difference of the arrival times of correlation peaks, which correspond to dibits occupying the same position in the [4 4 5 5] sequences of dibits in the two adjacent servo bands. However, during normal operation of the skew-following loop, there is an inherent ambiguity equivalent to the length of a servo frame, e.g., 152 μm for one servo format. This ambiguity arises from the skew estimate being obtained as the difference of the arrival times of correlation peaks, which correspond to dibits occupying the same position in the [4 4 5 5] sequences of dibits in the two adjacent servo bands, without distinguishing whether they belong to two servo frames encoding the same LPOS symbol or different LPOS symbols.
This ambiguity leads to a drawback where a reliable skew estimate is limited to the interval [−d, d], and that controlling the skew to values around ±d μm+K×2d μm, where K is an integer, poses a serious challenge, as the measured skew during skew-following might toggle between values around +d and −d. In case a perfect alignment between servo readers and servo frames is required, i.e., the head is to be kept perpendicular to the tape, the skew is controlled to around zero and the above mentioned ambiguity does not represent a problem.
In future tape drive generations, however, reliable read-after-write operation in the presence of offsets in the alignment of the head modules, as well as tape dimensional stability considerations, and track-following disturbances arising from compressional tape vibration modes, will dictate implementations of the skew-following loop, for which the reference skew for skew-following control may be in a wide range of values, well beyond the conventional limits of ±d. Furthermore, the conventional method leads to the generation of only one skew estimate per servo frame, which is not sufficient to adequately suppress coupling effects between skew- and track-following loops, and to avoid non-negligible estimation delays in skew-following control loops.
Therefore, a robust and reliable skew estimation method that allows skew-following control with skew reference values that may be selected in a wide range, and high rate of generation of skew estimates that are not limited to the interval [−d, d] would be highly beneficial.